1. Field of the Invention
The invention disclosed and claimed herein generally pertains to a driver apparatus for transmitting data signals through a digital I/O communication channel, wherein higher frequency signal components experience substantially more attenuation then lower frequency components. More particularly, the invention pertains to apparatus of the above type wherein pre-emphasis is used in transmitting a signal, so that the energy levels of the higher frequency components are increased as the signal travels through the channel. Even more particularly, the invention pertains to an apparatus of the above type wherein the amount of pre-emphasis is determined by the output impedance of the driver, and such impedance can be either constant or adjustable, selectively, and the driver can implement different levels of pre-emphasis.
2. Description of the Related Art
Typically, I/O communication channels, such as are used for off-chip communications and have a driver and a receiver, exhibit a frequency response similar to that of low pass filters. As a result of this, the higher frequency components of a transmitted data signal tend to experience larger amounts of attenuation than the lower frequency components. This degrades the transition times of the transmitted signal, and in extreme cases leads to errors in the data communication.
Pre-emphasis is a means of mitigating the signal degradation caused by the limited bandwidth of the channel. During pre-emphasis, a transmitted signal is pre-conditioned such that the relative energies of the transmitted signal after traversing the channel are back in line with those of the original signal, thus equalizing the frequency characteristic of the channel. Since the frequency response of a channel changes with different channels and conditions, a driver must be able to implement different levels of pre-emphasis, so that it can be used to transmit data over a range of channel conditions.
U.S. Pat. No. 7,212,035, to Dreps et al., describes an I/O driver that can be configured to provide the requisite different levels of pre-emphasis. This is achieved by adjusting the driver output impedance based on the data pattern being transmitted. The changing output impedance results in a change in the output voltage, which in turn results in higher frequency energy content in the transmitted signal. This capability also indirectly enables the driver to be used for applications requiring data to be transmitted over longer channel lines that are extremely lossy, wherein a high signal voltage swing is required to move the data through the channel.
In some situations, signal reflections caused by the mismatch between driver output impedance and channel impedance are of small enough concern that the resulting signal degradation can be tolerated. However, in other situations pertaining to higher speed communication links, such reflections are not generally permissible, and require both impedance matching and configurable pre-emphasis. As used herein, configurable pre-emphasis refers to the ability to adjust or set a driver to provide the desired level of pre-emphasis. U.S. Pat. No. 6,930,506, to Cranford, Jr. et al, describes a driver that can be configured to implement different levels of pre-emphasis, while maintaining a constant output impedance. Accordingly, the Cranford disclosure could be used to meet the above communication link requirements, where impedance mismatch is a problem. A drawback of the solution described by Cranford, Jr. et al, however, is the limited range of output impedances that is achievable by the Cranford driver. For example, it would not be practical to use the driver of Cranford in the same range of output impedances that can be achieved by the driver of Dreps et al. This limited range of output impedances indirectly limits the voltage swing of the transmitted signal.